In prior nuclear magnetic resonance imaging systems, spatial properties were determined by the application of a magnetic field which varied over the sample. Either the magnetic field was caused to be spatially dependent by design or by the addition of gradient coils, or a radio frequency field was employed which varied in space. The spatial distribution of spin density was then determined by either recording the signal strength from a selected region, or by recording the temporal evolution of transverse spin magnetization in a field gradient.
Problems with the above noted approaches are that linewidths and chemical shifts complicate the image process. These complications arise from: rapidly decaying signals due to very broad lines typical of solids; off-resonance effects during radio frequency pulses; and the necessity of differentiating between resonance offset which originate from chemical shifts and the transmitter offset, and resonance offset which originate from the applied magnetic field gradients. An additional problem, particularly for solids, is that these spatially dependent fields cause the line-narrowing sequences to deteriorate across the sample. This limits the spatial resolution of the sequence.
In U.S. Pat. No. 3,789,832 (Damadian) a method for imaging is described in which the spatial localization is achieved by means of a spatially inhomogeneous static magnetic field; the signal is obtained just for those spins in the small homogeneous region of the static field, and this sensitive point is slowly steered throughout the specimen.
In U.S. Pat. No. 4,301,410 (Wind et al.), a method for spin imaging solids using NMR spectroscopy is disclosed. According to the method, the sample is rotated about an axis at a particular angle to the NMR static external magnetic field. A magnetic field gradient with a spatial distribution which is related to the sample spinning axis is synchronously rotated with the sample. Data are then collected while performing a solid state NMR line narrowing step. The phase relation between the sample rotation and the field gradient rotation is then changed on a step-by-step basis to map out an image of the object.
In U.S. Pat. No. 4,609,872 (O'Donnell), a method for nuclear magnetic resonance imaging of fluid flow, particularly for blood flow, is disclosed. The method uses multiple-echo phase-constant sequences of signals both in the magnetic field gradient in the direction in which fluid flow is to be determined and in the radio frequency magnetic field utilized with the magnetic field gradient.
In U.S. Pat. No. 4,654,593 (Ackerman), a method for nuclear magnetic resonance imaging with a nonmagnetic moving object. The object to be analyzed is positioned in a field of a radio frequency excitation coil and a magnetic field of a nuclear resonance spectrometer. The object is of a low conductivity so as to be substantially transparent to electromagnetic radiation at the nuclear magnetic resonance frequency. The nonmagnetic object is subjected to periodic motion while transverse magnetization is generated. At least one phase-encoding magnetic field gradient pulse in at least one specified direction to the moving object, which is of sufficiently short duration so that the object does not move appreciably while the pulse is on, is then applied. The magnetic field gradient is then turned off and a free induct ion decay signal is detected. The steps are then repeated using appropriate increments in the intensities of the field gradient and an appropriate processing of the signals is performed.
In U.S. Pat. No. 4,947,120 (Frank), a method for nuclear magnetic resonance imaging of blood flow is disclosed. Flow induced phase shifts are distinguished from systematic phases produced during image formation, thereby enabling the separation of flowing and stationary components.
A method of solid state NMR imaging of stationary specimens employing pulsed magnetic field gradients and coherent averaging to improve spatial resolution is presented in a patent application (Navy Case Number 72,761) and in J. B. Miller, D. G. Cory, and A. N. Garroway, Chem. Phys. Lett., 164, (1989).
Surface coils for NMR imaging may be designed as aggregates of smaller, non-interacting coils, as shown in P. Mansfield, J. Phys. D (Appl. Phys.), 21, 1643 (1988).
Solid state NMR imaging has been accomplished with an inhomogeneous surface coil and a static sample, as in J. B. Miller and A. N. Garroway, J. Magn. Reson. 77, 187 (1988) and also J. B. Miller and A. N. Garroway, J. Magn. Reson. 85, 432 (1989).